Substitution Jigs Speed ServiceOctober 1957 Radio & TV News Article

Wax nostalgic about and learn from the history of early electronics.
See articles from Radio &
Television News, published 1919 - 1959. All copyrights hereby acknowledged.

Sure, you do it all the time, but at some point either someone suggested and/or showed you how to clip substitute
components into a circuit in place of a suspected bad component while troubleshooting or maybe during the design
or tuning process. Miniature circuits of today pretty much rule out using alligator clips to patch in a part, but
sometimes it is possible to use miniature J clip leads or even pin probes to make the connection long enough to
take a measurement. Since I do a fair amount of circuit work on other-than-RF applications, I often find myself
using short insulated, stranded wires with mini alligator clips on each end to make connections.

Substitution Jigs Speed Service

By B. Van Sutphin

Fig.1. Common, single-component jigs used in service, referred to
in text.

Fig. 2. Clip leads can also be fixed to elaborate components such
as potentiometers, i.f. cans, printed circuits.

Fig. 3. Two silicon rectifiers in holder can be hooked up in half-wave,
full-wave, doubler, or other configurations for power-supply tests.

Fig. 4. This detector jig provides an audio signal from a modulated
generator which does not have a separate audio jack.

Fig. 5. Trace of flat band of a sweep generator, taken through detector
probe.

The use of clip leads for temporary test hook-ups can be extended
to provide even more time-saving.

In the service shop, the accent must be on convenience and speed.
If time can be saved, that means more income per hour of work.

Most shops keep a collection of clip leads for temporary connections
to speaker voice coils, phono motors, and other points. These are simply
lengths of insulated wire with alligator clips at each end. In addition,
special jigs are often made up for connection problems that come up
again and again. Typical examples are jigs to fit the speaker sockets
of popular auto radios with leads to connect to the shop test speaker
and special jigs for feeding the sweep-generator output to a TV receiver.

This idea can be extended even further. Fig. 1 shows some single-part
jigs convenient in radio and TV servicing. Included are two common filter
capacitors, a special-value 1000-volt capacitor for testing in damper
circuits, a general-purpose 600-volt test capacitor, two resistors of
common value, and three low-value ceramic capacitors for general-purpose
testing. Although not shown in Fig. 1, a video-detector diode jig is
also convenient. The units shown are constructed by adding alligator
clips to each end of the individual parts. For maximum safety, insulated
clips can be used if they are available.

With a collection of these aids, part substitution is simply a matter
of connecting two clips. This system is much safer than trying to hold
a part in place while watching a TV screen and it is much simpler than
"tacking" new parts in place with solder. Think of the ease of substituting
new parts when the customer asks for a quick estimate!

In most cases, substitution can be made by clipping the replacement
directly across the suspected part. Of course, this does not apply when
the suspected defect is a short in the original part. In those cases,
you would have to disconnect one side of the original part and then
clip the new one in place, but even here some time is saved.

As shown in Fig. 2, this idea can be carried further yet. Clips can
be mounted on potentiometers, i.f. transformers, and even complete printed
circuits, provided there are not too many leads. The "can" shown in
Fig. 2 is a 4.5-megacycle sound take-off coil common in many receivers.
The printed circuit with three leads is a vertical integrator network,
complete with built-in blocking capacitor (Centralab PC-101). The blocking
capacitor is important so that the network can be used in more receivers
- this one can replace the original network whether it uses a blocking
capacitor or not, and whether it is a printed circuit or uses separate
components.

In Fig. 2, the printed circuit with four leads is a complete network
for coupling the plate of a triode audio amplifier to the grid of an
output stage.

If a number of these printed-circuit jigs are used, it is a good
idea to draw the schematic of each one on heavy paper and then paste
it to the side of the printed circuit. Use ink for drawing the schematic,
and then coat the drawing with spray plastic to protect it.

The twin-resistor jig at the upper left-hand portion of Fig. 2 is
a pair of 100,000-ohm resistors in series. This is for insertion in
discriminator or ratio-detector circuits during alignment. This is much
simpler than soldering individual resistors in place each time. "

The block-like device in Fig. 3 is a pair of silicon rectifiers (Sarkes
Tarzian M-500) in a standard holder. This unit can be used as a single
half-wave rectifier, a high-voltage half-wave rectifier (two rectifiers
in series, useful up to 260 volts), or as a two-section rectifier in
a voltage doubler. It can be used for temporary substitution in new
sets using silicon rectifiers or in older sets that originally used
selenium rectifiers, whenever it becomes necessary, in receivers of
this type, to make checks on the operation of the power supply.

Fig. 4 shows a detector circuit for obtaining an audio signal from
a signal generator that does not have an audio output jack. Only two
parts are used: a 1N34 crystal diode and a 470,000-ohm resistor. The
unit is constructed just like the twin-resistor jig shown in Fig. 2.
Connect the signal-generator output to" the input of the detector, set
the signal generator to any convenient frequency, and the audio signal
will appear across the 470,000-ohm resistor. This audio signal can then
be fed to the circuit under test. Its strength can, be adjusted with
the r.f. attenuator on the signal generator.

A similar unit using a 1N82 high-frequency diode "and a 4700-ohm
resistor is handy for checking the outputof a sweep generator to be
sure that it is reasonably constant over a particular range. If the
sweep generator does not have a terminating resistor at the output of
its cable, connect one having a resistance value equal to the characteristic
impedance of the output cable. This will prevent standing waves from
appearing because of possible impedance mismatching. Connect the sweep-generator
output to the detector input, and connect the scope input to the detector
output of the generator.

If the sweep-generator output is constant over the particular range,
the trace on the scope will be similar to that shown in Fig. 5. Any
humps or dips in the trace indicate that the sweep generator output
is not constant over the range. (Incidentally, it is well to check the
detector on various ranges first with a sweep generator known to have
constant output. This is to prevent possible resonance effects in the
detector circuit from giving misleading results. This is particularly
important on the higher TV channels.)

The jigs described here are those the author has found most useful.
Other service technicians may think up their own to fit particular problems
that come up again and again. Technicians doing warranty work on a specific
line of equipment will probably find special units with parts that frequently
fail very useful. Almost any part can be put in a jig like this. Perhaps
the only exceptions are parts used in high-frequency circuits. For example,
substitution of a new diode in a u.h.f. converter circuit cannot be
done this way. You still have to clip the leads and solder the part
in place.